As a method of controlling a torque of an alternating current (AC) electric motor, there are known a pulse width modulation (PWM) control in which a pulse width of an electric current is controlled based on a vector control, or a rectangular wave control in which rotation of the AC electric motor is rotated and driven by applying a rectangular wave voltage. In addition, as a control for improving an output power in a so-called flux weakening area, there is known a voltage phase control in which a torque of the AC electric motor is controlled by controlling a voltage phase depending on a difference between a torque command value and an actual torque in a rectangular wave voltage control.
In the rectangular wave voltage control, it is possible to improve the output power in the flux weakening area. However, control responsiveness is degraded. For this reason, when the rotation number of the electric motor changes abruptly, a control response of the applied voltage may be delayed, and an output electric current may be distorted.
In JP 2006-320039A, in the rectangular wave voltage control, degradation of the control responsiveness is prevented by controlling a voltage phase depending on a difference between a torque command value and an actual torque and correcting a voltage amplitude depending on a change rate of the rotation number of the electric motor when the rotation number of the electric motor abruptly changes.